You get it from throwing 4 high level rare/exotic level staves in the mystic forge (it's a random thing like most of the cheap, named exotics) OR you could have gotten it from the ancient karka event chest (another random thing).

It's a Mystic Forge. I was lucky to get it from 4 level 80 rares. Currently going for about 9g 50s, or so. I don't need the gold in a hurry so am thinking of saving it for my Guardian. It is a HUGE scepter.